Spurred on by increasing oil prices and growing concerns over environmental pollution, the use of fuel cells has sparked global interest as an alternative to fossil fuel and combustion technologies. Fuel cells are attractive for a number of reasons, e.g. low pollution, high energy efficiency, fuel flexibility, high quality power output, quick response to load fluctuations, excellent heat recovery characteristics, quiet operation, etc. Their high energy efficiency and low pollution partly derive from the use of a clean fuel source, e.g. hydrogen, methanol, etc.
There are many different types of fuel cells. Of particular promise in a broad range of applications is the polymer electrolyte membrane fuel cell (PEMFC), otherwise known as proton exchange membrane fuel cell. In a PEMFC as illustrated in FIG. 1, a layer of ion-conducting polymer electrolyte membrane 11 is sandwiched between a positive electrode, a cathode, and a negative electrode, an anode. Typically, the polymer electrolyte membrane measures from 20-200 μm in thickness. Each electrode lies adjacent to a catalyst layer, represented by 12 and 13; these catalyst layers speed up the oxidation and reduction of gases, e.g. oxygen and hydrogen, on both sides of the electrolyte membrane. Supporting layers 14 and 15, lying adjacent to catalyst layers 12 and 13, ensure effective diffusion of each reactant gas to the catalyst and even distribution of gases across the electrode surfaces.
A pair of gas-impermeable, bipolar plates 16, adjacent to the outer surface of supporting layers 14 and 15, serves as a gas flow field and current collector. To provide a gas flow field, plates 16 contain groove(s) along which reactant gases can travel after their introduction into the fuel cell. Each plate also acts as a current collector such that electrons produced by the oxidation hydrogen can be conducted 1) through the anode, through the adjacent supporting layer 14, and through anode plate 16, 2) travel through an external circuit, and 3) re-enter the fuel cell at the cathode plate 16.
The mechanism by which a PEMFC generates electricity is as follows. During fuel cell operation, a fuel such as hydrogen gas (H2) is distributed over the anode and reacted with catalyst layer 12 to generate protons and electrons. This oxidation reaction is represented by Chemical Equation 1 below.2H2→4H++4e−  <Chemical Equation 1>
The hydrogen ions, or protons, then penetrate the polymer electrolyte membrane 11 and travel towards the cathode while the electrons are conducted through an external circuit to the anode. At the cathode, an oxidant such as oxygen (O2) combines with electrons from the anode and undergoes reduction to oxygen ions (O2−) and reacts with the protons to form water, heat, and electricity. The above reactions are illustrated in Chemical Equation 2 below.O2+4e−→2O2−2O2−+4H+→2H2O  <Chemical Equation 2>
The polymer electrolyte membrane 11, which can be a cation exchange membrane, serves several important functions in a PEMFC. Namely, it functions as an insulator between the anode and cathode, an ion conductor between anode and cathode, and a separator for the fuel and oxidant. A polymer electrolyte for fuel cells should therefore have (a) low resistance to protons and electrons, (b) sustained selective permeability to hydrogen over a wide temperature range; (c) electrochemical stability; (d) enhanced conductivity; (e) ability to maintain separation of reaction products; and (f) robust chemical, mechanical, and dimensional properties suitable for a stack environment.
A leading polymer electrolyte commonly used in fuel cells and other applications is known by the trade name of NAFION (Dupont Co. Ltd.). NAFION is a perfluoronated membrane that is fabricated by melting tetrafluoroethylene and perfluorovinyl ethersulfonyl fluoride together, shaping the mixture, and then hydrolyzing the metal to yield the ionic sulfonate form. Its sulfonic acid group serves as an ion exchange group and the copolymer of tetrafluoroethylene and perfluorovinylether acts as a base. NAFION has high proton conductivity, approximately 0.1 S/cm at 25° C., under hydrated conditions, in which the amount of sulfonyl group (—SO3H) that dissociates is more than 20 wt % of the total weight of the polymer. Among single polymers, NAFION is also considered the most stable with respect to its mechanical, chemical, electrochemical properties. While NAFION has many good qualities in the context of PEM cells, its limitations have presented a number of problems.
For instance, NAFION is susceptible to dehydration resistance. Since NAFION transports protons with the aid of water, its proton conductivity suffers when the membrane is not sufficiently hydrated. If fuel cell operation were feasible under low humidity conditions, the weight and volume of the humidifier in fuel cell designs can be reduced to enhance total cell efficiency.
NAFION is also limited to operating temperatures below 100° C. since higher temperatures can result in low proton conductivity, dehydration resistance, and degradation of the membrane. There is however a great benefit to developing a fuel cell that can operate at higher temperatures. High operating temperatures can accelerate reactions in the fuel cell, thereby promoting system efficiency, and avoid or minimize carbon monoxide poisoning of the platinum catalyst(s). Therefore, the ability to operate at high temperatures is invaluable for the development of a medium or large PEMFC usable in household appliances, electric vehicles, and other applications.
To overcome the above technical limitations, efforts have been made on developing a variety of alternative membranes, none of which has demonstrated sufficient advantages to replace NAFION as the membrane of choice. In an attempt to overcome NAFION's limitations at low humidity and high temperatures, Staiti et. al. and Tazi et. al. impregnated NAFION with phosphotungstic acid and silicotungstic acid/thiophene, respectively. See, P. Staiti (2001) Materials Letters 47: 241-246, and B. Tazi et al. (2000) Electrochimica Acta 45: 4329-4339. In this manner, Staiti et. al. and Tazi et. al. were able to increase proton conductivity and hydration levels at temperatures approaching 120° C. by using these hydrophilic heteropolyacid compounds to prevent evaporation of moisture. However, this method has its drawback; the soluble additives tend to leach out of the electrolyte membrane structure during fuel cell operation, which thereby undercuts their utility.
Although water is a byproduct of fuel cell reactions and indispensable to the function of cation exchange membranes, the dependence on water to conduct protons in the electrolyte membrane has its disadvantages. Since water is a Brönstead-Lowry base with a high dielectric constant, sulfonyl groups (—SO3H) tend to dissociate quite readily from the NAFION membrane. As such, the use of an organic solvent of low volatility as a proton acceptor within the electrolyte membrane has been considered. Other electrolyte membranes employing phosphoric acid, imidazole, butyl methyl imidazolium triflate, or butyl methyl imidazolium tetrafluoroborate have also been contemplated, each with its own inherent disadvantages (R. Savinell, et al., J. Electrochem. Soc., 141, L46 (1994), K. D. Kreuer, A. Fuchs, M. Ise, M. Sapeth, J. Mater. Electrochem. Acta, 43, 1281 (1998)).
Other strategies to overcome the moisture dependency are directed to an anhydrous method of conducting protons across the electrolyte membrane. To this end, solid acids such as cesium hydrogen sulfate (CsHSO4), or zirconium hydrogen phosphate (Zr(HPO4)2) have been added to electrolyte membranes. See S. M. Haile, D. A. Yoysen, C. R. I. Chisolm, R. B. Merle (2001) Nature 410: 910.
There are several problems associated with the above technique. First, these large particles of high-density inorganic additive become non-uniformly dispersed on the NAFION membrane as the NAFION solution, in which CsHSO4 or Zr(HPO4)2 is suspended, dries. Second, such additives do not adhere well to the polymer matrix and their addition has the effect of weakening the membrane. See U.S. Pat. No. 5,919,583. Additionally, the instability of these inorganic additives makes them poorly suited to the role of an ionic conductor in the context of fuel cells. For example, the acid will dissociate into Zr4+ ions which find their way to the hydrophilic domain or surface domain of a hydrated NAFION membrane, leaving only about 20 wt % of intact Zr(HPO4)2 on the membrane.
In light of the above, there is a need in the art for an electrolyte membrane that has good ionic conductivity even at elevated temperatures, the ability to maintain separation of reaction products; and robust chemical, mechanical, and dimensional properties suitable for a stack environment.
The present invention provides a way to overcome the aforementioned problems by uniformly dispersing nanoscopic dendrimers on an acid-treated surface of the hydrophilic domain of cation exchange polymer. An improved electrolyte membrane that exhibits high ionic conductivity and low fuel permeability even at low humidity and high temperatures is thereby obtained.